The detection of gas-phase molecules using Raman spectroscopy was at one time considered impossible. F. Calegari, et al., Molecular rotovibrational dynamics excited in optical filamentation, Optics Letters, Vol. 33, No. 24, 2922-2924, (Dec. 15, 2008) [3], demonstrated the use of laser filaments in argon to temporally shorten and spectrally broaden a femtosecond pump beam. The fundamental stretching mode of H2 was detected, implying a pump pulse with structure smaller than 8 fs. Calegari measured his pulse width as 6.8 fs. Calegari reports detecting gas phase molecules such as CO2 and H2 at superatmospheric pressure. Calegari's pump and probe beams were generated by beam-splitting a single pulse with a variable delay line in the probe beam, and were thus both of fs duration. Calegari generated a time-domain sequence as a function of the pump-probe delay, and Fourier transformed that sequence to produce a frequency spectrum.
A team including the present inventors demonstrated that such molecules could be detected using an air-based filament. See Odhner, et al. Physical Review Letters, 103, 075005 (2009) [1]. Nitrogen and oxygen in air were successfully detected, using a 45 fs pump pulse shortened by filamentation, and the 8 fs hydrogen fundamental stretching mode was also detected. Odhner et al. used a 1.5 ps probe beam crossing the pump beam at a small angle, enabling a one-shot spectrum.
Further work by the same team is described in J. H. Odhner, et al., Filament Based stimulated Raman spectroscopy, SPIE Photonics West Conference 2010, Vol. 7582, 75820M (2010) [2].
[4] P. J. Bustard et al., Amplification of Impulsively Excited Molecular Rotational Coherence, Physical Review Letters, 104, 193902 (2010) [4], coupled a long (ns) probe pulse to the impulsively excited [rotational] Raman coherence generated by a short pulse. In their paper they report the sustenance and amplification of the coherent properties of the material excitation over much longer durations than would be expected without the long probe pulse.
The laser systems (oscillators and amplifiers) used in Refs. [1] to [4] are commercially available technology. The filamentation process utilized in Calegari, et al. (2008) [3] and Odhner, et al. (2009 and 2010) [1] and [2] is not commercially available technology, though the process is widely used, and is highly dependent on the input parameters of the laser beam (pulse duration and energy, as well as spatial characteristics). The choice of input parameters for filamentary pulse self-shortening is often highly specific to the working conditions of the technique.
Nonlinear pulse shortening during filamentation in noble gases as used in Calegari, et al. is a well-known and well-researched phenomenon, whereas the implementation of filamentary propagation for pulse shortening in air, as described in Odhner, et al. [1] and [2] is relatively new and is not well researched. The use of air as a propagation medium is not obvious, as it was previously thought that the molecular nature of the medium had a detrimental effect on the pulse-shortening phenomenon.
The present inventors have now discovered that filamentary propagation in air makes possible Raman spectroscopy at considerable distances from the exciting lasers, and that much higher probe pulse intensities than previously proposed make it possible to access a novel regime of coupled non-linear effects, in contrast to the single non-linear effect used in conventional stimulated Raman scattering or impulsive stimulated Raman scattering.